Gen 6 digit GMT crystal is 1.8mm thick with a 2.7mm gasket
IDK how thick a CF crystal is but if it is 2.0mm then you would need a 0.2mm shorter gasket (assuming the cyclopses are identical mag) to keep the same appearance.
Rather than approaching it that way, I would just buy extra gaskets from Gary (Clark) and have an assortment available to the person doing the work. The gasket heights that work most often in GMTs and Subs are 2.5, 2.7 and 2.85. It never hurts to have a 2.3 and a 3.0 on hand too. I test fit the one I think it should need and if it isn't an immediate sure thing, I'll try a shorter or taller gasket. Sometimes I take pics and make a side-by-side collage to make certain I can live with my choice. Clark's gaskets are good quality and only like $4. If buying extras, ask Gary to make a special listing for you so you don't pay extra shipping.
Also, crystal gaskets are quite easily shortened to make a custom height by circularly sanding them on a flat bench with sandpaper. Use 400 grit or 320. Use the crystal as a gasket holder (or a junk one) and rotate grip often to distribute pressure evenly. Measure and/or test fit often so you don't take too much off. Don't use black sandpaper, it can stain the gasket.